1. Field of the Invention
The present invention relates generally to a method and apparatus for modulating symbols in a wireless communication system, and in particular, to a method and apparatus for efficiently modulating symbols in an Orthogonal Frequency Division Multiplexing (OFDM) system, and a transmitter using the same.
2. Description of the Related Art
With the ongoing development of the communication industry and the increasing user demands for packet services, there is an increasing need for communication systems that are capable of efficiently providing the packet services. The conventional communication networks, as they have been developed to mainly provide voice services, have narrower data transmission bandwidths and higher service charges. To solve these problems, extensive research is being conducted on Orthogonal Frequency Division Multiplexing (OFDM), which is the typical example of the broadband packet transmission scheme.
OFDM, the typical multi-carrier transmission scheme of overlapping multiple orthogonal subcarriers, converts a serial input symbol stream into parallel streams and modulates each of the parallel streams with multiple orthogonal subcarriers before transmission. It is known that the OFDM scheme can provide an efficient platform for high-speed data transmission using its robustness against multipath fading.
The Orthogonal Frequency Division Multiple Access (OFDMA) system recently attracting public attention, which is an OFDM-based multiple access system, divides the frequency domain into subchannels each having a plurality of subcarriers, and allocates the subchannels to the users individually to perform resource allocation taking both the time and frequency domains into account, thereby accommodating multiple users with the limited frequency resources. Herein, the use of the term “OFDM system” will be construed to include the OFDMA system.
The OFDM scheme, as it divides an input data stream into NS subcarriers before transmission, can reduce NS times the relative multipath spread for the symbol period by increasing the symbol interval NS times. Because transmission of the OFDM symbols is processed on a block-by-block basis, while the OFDM symbols are transmitted over multiple paths, the currently transmitted OFDM symbol may receive interference from the previously transmitted OFDM symbol. As is well known, a scheme for inserting a guard interval between consecutive blocks is used to efficiently cancel the inter-OFDM symbol interference, or Inter-Symbol Interference (ISI). The guard interval is selected to be longer than the expected delay spread so that the multipath component from a previous OFDM symbol will not interfere with a current OFDM symbol.
For the OFDM symbols, a Cyclic Prefix (CP) insertion scheme for copying a partial interval of an OFDM symbol and cyclically-concatenating the copy to the guard interval is used to prevent Inter-Channel Interference (ICI). The ‘CP insertion’ refers to an operation of copying a block of, for example, a length NCP at the back of an OFDM symbol and filling the guard interval with the copied block. Even though a subcarrier delay occurs by the CP insertion, an integer period is maintained in a Fast Fourier Transform (FFT) interval, guaranteeing the orthogonality, and only a phase rotation caused by the delay occurs, making it possible to prevent the ICI.
In the OFDM system, a symbol modulation scheme parallel-converts serial input data into a number of parallel data streams equal to the number of subcarriers, and modulates each of the parallel-converted data streams with its associated subcarrier. Subcarrier modulation/demodulation can be realized by Discrete Fourier Transform (DFT) and Inverse Discrete Fourier Transform (IDFT). Because the DFT and IDFT have the higher hardware complexity and the higher calculation, they can be realized using Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) algorithms, respectively, to perform the high-speed operation while reducing the complexity and the calculation.
The CP insertion is performed in a symbol modulation process of the OFDM system, and for the CP insertion, an output of IFFT is stored in a buffer of a transmitter. In this way, for the CP insertion, the modulation process of the OFDM symbol should store the IFFT output in the buffer until the samples disposed at the back of the IFFT output are output, inevitably causing a delay. In addition, for hardware, there is a need for a buffer for storing all the output of the IFFT. Therefore, a reduction in the delay and the required buffer capacity (or required buffer size), if possible, can improve performance of the transmitter in the OFDM system. That is, for the performance improvement of the transmitter in the OFDM system, there is a need for an apparatus and method capable of reducing the delay and the required buffer capacity.